1. Field of the Invention
This invention relates to relay circuits and, more particularly, to relay circuits which utilize photosensitive silicon-controlled rectifiers as active switching elements which are triggered into conduction only when the value of the AC voltage being controlled is within predetermined limits of zero volts.
2. History of the Prior Art
A number of circuits have been devised in the prior art to provide an AC relay consisting of a pair of inverse-parallel-connected photosilicon controlled rectifiers which are triggered into conduction by light emitting diode light sources optically coupled thereto through a transparent dielectric path. The light emitting diodes are in turn energized by an input signal and provide light output to impinge on the photosilicon controlled rectifiers to cause them to conduct and complete an output circuit between an AC power source and a load. Such relay circuits are widely used for the control of AC loads by computer generated logic signals such as are found in microcomputer based control systems. The optical coupling between the input signal circuit and the load circuit provide excellent electrical isolation between the logic signals and the AC loads. In addition, the photosilicon controlled rectifier relay circuit requires few components for its implementation. This simplicity permits the use of hybrid circuit construction techniques to provide solid state AC relay circuits which may be inexpensively mass produced in a very small package size.
For most applications, it is desirable to have an AC relay which switches on only at those points in time when the value of the AC voltage being controlled is close to zero. This eliminates a great deal of the transient problem which occurs if the relay is switched on at an instant when the AC voltage being controlled has a substantial value. This type of circuit action is known to those skilled in the art as zero voltage switching, although the actual switching action may occur within a range of voltages around zero. In general, circuits of the prior art providing zero voltage switching for AC relays using photosilicon controlled rectifiers possess a large number of components in order to achieve acceptable performance. Such circuits are expensive and thus obviate one of the main benefits of their use. In an effort to provide a zero voltage switching circuit using a minimum number of components, prior art designs have employed a transistor having its collector and emitter connected to the gate and cathode, respectively, of the photosilicon controlled rectifier. The transistor base is connected through a series current-limiting resistor to the anode of the silicon controlled rectifier so that the anode to cathode voltage of the silicon controlled rectifier controls the transistor. This circuit uses the transistor to clamp the gate to cathode circuit of the photosilicon controlled rectifier whenever the voltage across the silicon controlled rectifier anode-cathode circuit exceeds a predetermined value to prevent the silicon controlled rectifier from being triggered into conduction.
Unfortunately, the silicon controlled rectifier gate clamping circuit described has the effect of significantly increasing the level of light required to trigger the photosilicon controlled rectifier into conduction. Such an increase requires an increase in the light emitting diode current supplied by the input signal source to the point that prior art AC relay circuits employing zero voltage switching configurations could not be used in such an arrangement and be directly controlled by computer generated logic signals. These computer logic circuits are limited by the amount of current they can supply to drive the input of the AC relay circuit, so in order to work require additional amplifiers and power drivers to interface the prior art relay circuits to the computer logic.
For this and for other reasons, a new relay circuit including a pair of photosilicon controlled rectifiers connected in inverse parallel to provide full wave control of an AC voltage was devised and described in U.S. Pat. No. 4,339,670. This arrangement provides zero voltage switching by using the collector-emitter circuit of a transistor to shunt the gate-cathode circuit of the photosilicon controlled rectifier. The bases of transistors thus used with each silicon controlled rectifier are connected to each other by a single current limiting resistor. The emitter-base breakdown voltage of each transistor sets a predetermined zero voltage switching level and results in a zero voltage switching AC relay circuit which does not require high levels of input current for operation. Such a circuit functions well with computer generated logic signals.
Although this last described circuit provides improved performance, there are certain conditions in which some AC power sources generate abnormal amounts of electrical noise which, even using zero crossing circuitry such as described in the above-mentioned U.S. patent, causes the silicon controlled rectifiers to fail to turn on in response to input signals and cause so-called "dropout" half cycles.